Neurons located in the ventral medulla project to the thoracic intermediolateral cell column (IML) and modulate sympathetic activity to the cardiovascular system. Numerous putative neurotransmitters are found either alone or coexisting in cell bodies of IML-projecting ventral medullary neurons that are located in the midline raphe and paraphyramidal region. Substance P (SP), serotonin (5HT), and thyrotropin-releasing hormone (TRH) coexist in individual IML-projecting cells, Neurokinin A (NKA), another tachykinin peptide related to SP, may also be colocalized in these neurons. We propose that each of these colocalized neurochemical agents is released in the IML and affects the cardiovascular system either directly through interactions with IML neurons or indirectly by modifying the effects of another colocalized agent. To test this, we will: (1) obtain additional information on the chemical neuroanatomy of these 4 coexisting putative transmitter systems in IML-projecting ventral medullary neurons; (2) determine which of the colocalized neurochemical on their release; (3) asses the influence of the multiple colocalized putative transmitters and their interactions on cardiovascular control. The techniques to be used in these studies include: multiple fluorescence labeling of single cells with immunocytochemistry and retrograde tracers, in vitro receptor autoradiography, both in vitro and in vivo studies of the release of putative transmitters, and in vivo pharmacologic and hemodynamic studies in rats. These studies will provide information on the neurochemical and functional significance of the presence of multiple putative transmitters in IML- projecting neurons and provide insights into a possible neurochemical basis for discrete modulation and ultimately therapeutic manipulation of sympathetic input to the cardiovascular system. In addition, the information will provide more general insights into the neurobiology of colocalized neurochemical.